Add support for variable names in template destination filename

This pull request include what @joaoafrmartins  has suggested.

I also like the idea to add this functionality to the directory method in order to be able to recursively process templated directories.

In case you decide to go on this direction, is the directory method the properly place to add it? 
As we have ´copy´ and ´template´ methods, should we have ´directory´ and ´templateDirectory´ methods too?